Osteoarthritis (OA) of the carpometacarpal (CMC) joint is a debilitating disease, which afflicts with greater prevalence in the elderly female population than the male population. In their current study, applicants have focused on studying the form of the CMC cartilages. During this grant period, they validated the following hypotheses: female CMC joints are less congruent than those in males; female CMC joints have generally more compliant cartilage than males; cartilage is more compliant in compression in high load-bearing regions, than in low load-bearing regions of the OA CMC joint; and remodeling occurs with OA progression, leading to increased joint congruence in the end-stage disease. These findings are new, and they provide the needed information to explain the greater prevalence of OA in the female CMC joint. At present, there is little data on how laxity (a commonly diagnosed condition associated with the disease) affects the function of the CMC joint, and how the affected function can exacerbate the progression of CMC OA. It remains to be demonstrated that CMC laxity will result in smaller contact areas and higher contact stresses in congruent, and in non-congruent joints. Thus, this competing continuation application is focused on the form-and-function of CMC joints. The hypotheses to be tested in this study are: (H1) higher laxity (as measured with a custom-designed laxity tester) in the early stages of OA (as determined by radiologic, visual and histologic staging) is more common in females than in males; (H2) higher laxity promotes incongruent articulating positions (acquired incongruence) in the CMC joint, i.e., smaller contact area(s) and higher contact stresses, for certain relatively less stable activities of daily living (ADLs), e.g., tip and chuck pinch, but not for relatively stable ADLs, e.g., lateral (key) pinch; these changes would be exacerbated in intrinsically incongruent articular surfaces; and (H3) selective muscle strengthening (e.g., exercise) and selective tendon transfer procedures can be used to stabilize an unstable CMC joint. Along with their ability to determine ligament tensile properties, generate accurate anatomic data and test the thumb CMC joint with great accuracy, this project will extensively use the newly developed multibody computer models which may open unique perspectives in the study of CMC joint OA.